A number of molybdenum bismuth-based catalysts for oxidation reactions of organic compounds are known Examples of such catalysts include P.Mo.Bi catalysts as disclosed in JP-B-36-3563 (corresponding to U.S. Pat. No. 2,941,007) and JP-B-36-5870 (corresponding to U.S. Pat. No. 2,904,580) (the term "JP-B" as used herein refers to an "examined Japanese patent publication"); P.Mo.Fe.Bi catalysts as disclosed in JP-B-38-l7967 (corresponding to U.S. Pat. No. 3,226,422) and JP-B-39-3670 (corresponding to U.S. Pat. No. 3,171,859); Mo.Bi.Sb catalysts as disclosed in JP-B-39-10111; Mo.Bi.Pb catalysts as disclosed in JP-B-42-7774; and Mo.Bi.Cr catalysts as disclosed in JP-A-50-64191 (corresponding to U.S. Pat. No. 4,174,354) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). Further, several improvements have been proposed with respect to catalyst activity as disclosed, e.g., in JP-B-47-27490 (corresponding to U.S. Pat. No. 3,959,384), JP-B-54-22795 (corresponding to U.S. Pat. No. 3,984,477), and JP-B-60-36812 (corresponding to U.S. Pat. Nos. 4,600,541 and 4,377,534).
On the other hand, improvements in the yield of a desired oxidation product have been attempted through the improvements in preparation processes.
For example, JP-B-43-22746 discloses a method comprising adding a bismuth citrate aqueous solution to a molybdic acid aqueous solution; JP-A-53-l0388 discloses a method comprising mixing a molybdic acid aqueous solution with a solid bismuth compound and adjusting the pH of the slurry to 8 to 10; JP-A-53-l0387 and JP-B-55-l2298 (corresponding to U.S. Pat. No. 3,847,831) each discloses a method comprising mixing a molybdic acid aqueous solution with a solid bismuth compound while maintaining the pH of the mixture at a definite level; and JP-B-59-51848 (corresponding to U.S. Pat. No. 4,418,007) discloses a method comprising simultaneously adding a bismuth salt aqueous solution and aqueous ammonia to a molybdic acid aqueous solution having a pH of from 6 to 8. JP-B-59-51849 (corresponding to U.S. Pat. No. 4,388,226) discloses a method comprising adding a bismuth salt aqueous solution to a suspension of a molybdenum compound; JP-A-55-13187 (corresponding to U.S. Pat. No. 4,212,766), JP-A-55-47144 (corresponding to U.S. Pat. No. 4,148,757) and JP-B-60-29536 (corresponding to U.S. Pat. No. 4,040,978) disclose methods comprising preliminarily preparing various molybdates; JP-B-52-22359 and JP-B-52-47435 (corresponding to U.S. Pat. No. 3,872,148) disclose methods comprising preliminarily preparing various bismuth compounds; and JP-A-62-23548 discloses a method comprising using bismuth oxide or basic bismuth carbonate as a bismuth source.
Thus, a number of methods comprising, for example, mixing a molybdic acid aqueous solution with a bismuth compound by a newly devised method or using a specifically selected bismuth material has been proposed.
As JP-B-58-8895 (corresponding to U.S. Pat. No. 4,018,712) shows, an unexpected economical advantage can be achieved by elevating the yield of the target product by 1% in a commodity chemical process (for example, oxidation or ammoxidation of an olefin). Thus, studies for improving catalysts, such as those described above, have been continuously carried out.
On the other hand, a method for preparing a catalyst comprising adjusting the pH of a molybdenum containing slurry and adding a chelating agent (refer to U.S. patent application Ser. No. 480,698 filed on Feb. 15, 1990 and Japanese Patent Application No. 63-210427) has been also proposed. Although the molybdenum bismuth-containing catalysts thus prepared have excellent properties, for example, activity, attrition resistance and ammonia combustion characteristics, the present invention aims at providing further improvements.